Conventionally, there is known a working machine disclosed in patent literature 1, for instance.
The working machine disclosed in patent literature 1 is provided with a boom cylinder; an arm cylinder; a hydraulic pump which supplies hydraulic oil to the arm cylinder; an engine which drives the hydraulic pump; a regeneration valve switchable between an opened state, in which hydraulic oil drawn out of a head side chamber of the boom cylinder is guided to a rod side chamber of the arm cylinder, and a closed state; and a controller which controls the regeneration valve to switch to the opened state when a combined operation of lowering a boom and pressing an arm is performed. According to the working machine having the above configuration, it is possible to utilize the position energy of the boom at the time of lowering the boom, as the energy for pressing the arm.
Further, the controller in patent literature 1 is configured to reduce the ejection flow rate of the hydraulic pump in accordance with supply of hydraulic oil from the boom cylinder to the arm cylinder through the regeneration valve during a combined operation. This makes it possible to reduce the workload of the hydraulic pump during a combined operation. Thus, it is possible to enhance the fuel consumption rate of the engine.
In the working machine disclosed in patent literature 1, however, it is impossible to sufficiently reduce the drive loss of the hydraulic pump (the engine) during a combined operation.
Specifically, in the working machine disclosed in patent literature 1, although the ejection flow rate of the hydraulic pump during a combined operation is reduced, a part of the ejection flow rate of the hydraulic pump may be surplus, even if the ejection flow rate of the hydraulic pump is reduced to a minimum flow rate. For instance, in the case where a flow rate obtained by summing up the minimum flow rate of the hydraulic pump and the regenerative flow rate with respect to the boom cylinder exceeds a flow rate required for the arm cylinder, the hydraulic pump ejects the surplus flow rate. In this case, the surplus flow rate ejected from the hydraulic pump is wasted as heat energy for opening a relief valve.